Thousands of demonstrators descended on Reykjavik, Iceland, on Monday, calling for Prime Minister Sigmundur David Gunnlaugsson to resign after the “Panama Papers” revealed his ties to a company in the British Virgin Islands.CreditCreditStigtryggur Johannsson/Reuters

LONDON — Iceland’s prime minister faced calls for his resignation and a large demonstration outside Parliament on Monday in one of the first repercussions of an enormous leak of documents about offshore shell companies and tax shelters from a secretive Panamanian law firm.

The prime minister, Sigmundur David Gunnlaugsson, insisted he would not resign after documents revealed that he and his wealthy partner, now wife, had set up a company in 2007 in the British Virgin Islands through the law firm. The documents also suggest that he sold his half of the company to her, for $1, on the last day of 2009, just before a new law took effect that would have required him as a member of Parliament to declare his ownership as a conflict of interest.

Mr. Gunnlaugsson said the leak contained no news, adding that he and his wife, Anna Sigurlaug Palsdottir, had not hidden their assets or avoided paying tax.

But the company, Wintris Inc., lost millions of dollars as a result of the 2008 financial crash that crippled Iceland, and the company is claiming some $4.2 million from three failed Icelandic banks. As prime minister since 2013, Mr. Gunnlaugsson was involved in reaching a deal for the banks’ claimants, so he is now being accused of a conflict of interest.

When asked by Swedish television journalists about Wintris before the publication of the leaks, Mr. Gunnlaugsson stormed out, saying that the journalists had obtained the interview “under false pretenses.” He and his wife then issued statements about “journalist encroachment” in their private lives and said they had done nothing wrong.

Many international officials who were listed in papers revealing the offshore accounts of wealthy individuals, including associates of President Vladimir V. Putin of Russia, defended themselves on Monday.CreditCreditBrynjar Gunnasrson/Associated Press

They began reporting Sunday on the leaks, now known as the Panama Papers, which have implicated a range of politicians, celebrities and sports figures, including close associates of President Vladimir V. Putin of Russia, President Petro O. Poroshenko of Ukraine, Prime Minister Nawaz Sharif of Pakistan, current and former members of China’s ruling Politburo and FIFA, the worldwide association for soccer. A barrage of angry responses and denials of wrongdoing have followed.

The leaked documents drew immediate attention Monday from officials in Washington, who could be interested in using them as a road map of diplomatic, financial and even criminal dealings around the globe that are connected to the United States and its financial system.

“We are aware of the reports and are reviewing them,” said Peter Carr, a spokesman for the Justice Department in Washington. Josh Earnest, the White House press secretary, declined to comment specifically on the documents, but said the United States remained a “leading advocate for increased transparency in the international financial system, and in working against illicit financial transactions and in fighting corruption.”

The leaks come at a time of populist anger against business and political elites and sharpening income inequality in Europe and the United States. The information could intensify that anger by suggesting that government officials are lining their pockets in secret and avoiding transparency and taxes on their assets.

The anger is particularly acute in Iceland, a tiny country of 330,000 people that was hammered by the financial crisis. When the bubble burst, its three private banks, which had become bloated, could not cover their short-term debts and repay depositors. Iceland suffered a deep economic depression, a stock market collapse and huge losses, requiring emergency loans from the International Monetary Fund and capital controls.

Iceland is recovering well but the anger persists, and Mr. Gunnlaugsson, of the center-right Progressive Party, may not survive the political turmoil.

Protests erupted on Monday evening in front of the Parliament building. A former prime minister, Johanna Sigurdardottir, said Mr. Gunnlaugsson would have to resign if he could not quickly regain the public’s trust. A former finance minister, Steingrimur Sigfusson, told The Guardian newspaper: “We can’t permit this. Iceland would simply look like a banana republic. No one is saying he used his position as prime minister to help this offshore company, but the fact is you shouldn’t leave yourself open to a conflict of interest, and nor should you keep it secret.”

Having at first denied ownership in Wintris, Mr. Gunnlaugsson said his joint ownership with his wife was an oversight and was corrected when it was pointed out to him in 2009. He also said Wintris was a holding company to invest his wife’s assets from a company sale, a distinction that made it legally unnecessary for him to declare it in any case.

Ramon Fonseca, the director of Mossack Fonseca, defended the firm and the legality of offshore companies, telling Reuters in a telephone interview that the firm had suffered a successful but “limited” hacking intrusion. Mr. Fonseca, the firm’s co-founder and until March a senior government official in Panama, said his firm had formed more than 240,000 companies, adding that the “vast majority” had been used for “legitimate purposes.”

In Moscow, which had warned previously of an investigation aimed at Mr. Putin, his spokesman, Dmitri S. Peskov, said the reports were intended to smear Russia before elections there and were an outgrowth of “Putinophobia.” He suggested that the International Consortium of Investigative Journalists, based in Washington, had links to the United States government and other “special services” and said the documents showed “nothing concrete and nothing new about Putin.”

The documents do not mention Mr. Putin but indicate that numerous close associates, including a cellist, Sergei Roldugin, a childhood friend of Mr. Putin, had assets of up to $2 billion stashed abroad.

British newspapers highlighted leaked information that showed Prime Minister David Cameron’s late father, Ian, had an offshore investment fund for his wealthy clients that avoided paying tax in Britain, in part by getting numerous residents of the Bahamas, including a part-time bishop, to sign its paperwork.

Asked if there was still any family money invested in the fund, a spokesman for Prime Minister Cameron, Helen Bower, said, “That is a private matter.” Mr. Cameron has been outspoken about what he has called the need to shut down offshore tax havens and is scheduled to host a summit meeting on the issue in May.

The repercussions extended to Argentina’s president, Mauricio Macri. Leaked documents showed he was a director of an offshore company, Fleg Trading Ltd., incorporated in the Bahamas in 1998 and dissolved in 2009. Mr. Macri, who was mayor of Buenos Aires before his election as president last November, did not disclose his connection to the company in asset declarations in 2007 or 2008. Mr. Macri’s office said in a statement that the disclosure had not been required because the president had no stake in the company.

Simon Romero contributed reporting from Rio de Janeiro, and Eric Lichtblau from Washington.

A version of this article appears in print on , on Page A8 of the New York edition with the headline: ‘Panama Papers’ Put Scrutiny on Iceland Leader’s Offshore Holdings. Order Reprints | Today’s Paper | Subscribe